1. Field of the Invention
The invention relates to a hearing aid together with a method for operating a hearing aid with an input transducer for receiving an input signal and converting it into an electrical input signal, a signal processing unit for processing the electrical input signal and effecting frequency-dependent amplification of it, and for generating an electrical output signal, and an output transducer for converting the electrical output signal into an output signal which the user can perceive as an acoustic output signal.
Due to the required small size of the devices and the small distance between the input transducer (in particular a microphone) and the output transducer (in particular an earpiece), the maximum amplification which can be achieved with a hearing aid is generally subject to narrow limits. In particular, if the amplification is too high an extremely disturbing feedback whistle arises. The feedback susceptibility of a hearing aid is frequency-dependent, and mostly affects the upper end of the frequency range which a hearing aid can transmit.
For many of those with damaged hearing, the problem thus arises that even when they are equipped with a hearing aid they can no longer adequately perceive certain frequency ranges. In the perception of speech this leads to certain sounds not being correctly understood, in particular consonants which in terms of speech have signal components which are in the high frequency signal spectrum. This particularly affects the so-called fricatives, which are named after the way they are articulated, for example “s”, “sh”, “v” or “z”.
A known way of compensating for the loss of hearing described is to transpose the frequency ranges affected into other frequency ranges, which can be better perceived. In performing a frequency transposition of this type, two main methods are distinguished: in the case of frequency displacement, a frequency range (e.g. 4 kHz-6 kHz) is shifted into another frequency range (e.g. 2 KHz-4 kHz). In contrast to this, in the case of frequency compression the frequency of the output signal is produced by multiplying the frequency of the input signal by a factor (e.g. 0.75). Often however, frequency compression is not effected with 0 Hz as the starting point, but only above a certain frequency (the knee point) e.g. 2 kHz.
Methods for frequency transposition in a hearing aid, and a hearing aid for carrying out a frequency transposition, are known from the publication, European patent application EP 1 441 562 A2.
Frequency transposition, in particular frequency shifting, has two main disadvantages: on the one hand in respect of the spectrum, corruption of the original spectral composition of certain consonants and other sounds and, on the other hand, the ability to distinguish different fricatives—which affects their perception—is significantly impaired.
From the prior art, methods of speech signal processing are known by which the vowels or consonants can be recognized in a voice signal. For example, German utility model DE 691 05 154 T2 discloses a method of this type with which a voice signal spectrum is analyzed for the purpose of determining peak and average values, which are compared with certain threshold values to recognize vowels and consonants.
Also, a method is known from U.S. patent publication No. 2009/0112594 A1 whereby pre-vowel consonants and post-vowel consonants are distinguished on the basis of acoustic models.
U.S. Pat. No. 5,014,319 describes a hearing aid in which a frequency analysis device classifies sounds and, for a frequency transposition, determines a transposition factor appropriate for the frequencies occurring in the sounds. In order to avoid the frequency transposition suppressing individual speech sounds, so that useful information is then lost, a device for the reconstruction of speech components is provided in this hearing aid. For this purpose, the start of each individual speech sound which is recognized is stored, and the residue which remains is in each case discarded. The actual frequency transposition is then achieved by outputting these speech start-sounds over an extended time.